Smoke simulation - how to make a slow fog
There are 2 ways and you may need to use a combination of both depending on how slow you want to slow it down:

1) Change the Time: Scale with the CUBE DOMAIN


2)
Slow down the frame rate. E.G. change 29.97 to 120 fps.  Unfortunately it's not a perfect solution- it will change the characteristics of your smoke and you'll basically have to tweak everything from the very beginning.